Conventional electronic ballasts are suitable for the operation of different types of gas discharge lamps, e.g. for the operation of different types of low-pressure discharge lamps. Units of this type are referred to as multilamp units or as intelligent electronic ballasts. In a multilamp unit, the operating parameters for those types of lamps are stored which can be operated by this unit. For example, different types of lamps differ in the lamp current necessary for their operation. The unit selects the operating parameters to be used as a function of the type of the connected lamp. To this end it needs to identify the lamp type beforehand.
Some of the conventional units identify the lamp type by evaluating the cold resistance of a filament in the lamp at every start. If gas discharge lamps are used, in which the lamp filaments are preheated, the warm resistance of a lamp filament at the end of the preheating phase can also be evaluated. However, it is possible for identification errors to occur. If the lamp type is changed, the preheating parameter set which is still present from the previous lamp is used for the newly connected lamp, with the result that it is not preheated in an optimum fashion. As a result the ascertained value for the warm resistance of the filament can be in a region which cannot be assigned uniquely to a lamp type. Identification errors may result. However, if a lamp is operated using the wrong parameter values, it may not ignite properly or the service life of the lamp is reduced.